The present invention generally relates to electron devices and more particularly to a single-electron device including ultra-fine conductive particles or so-called nanocrystals dispersed in an insulating film and a fabrication process thereof.
Since the discovery of quantized, stepwise change of conductance in a system of ultra-fine metal particles dispersed in an oxide film by Wilkins, et al. (Wilkins, R., Ben-Jacob, E., Jaklevic, R. C., Phys. Rev. Lett. 63, 1989, pp.801), intensive investigations are being made on a so-called single-electron device that uses the phenomenon of Coulomb blockade of electron. By using the Coulomb blockade of electron, it becomes possible to construct a device that achieves a switching operation based upon a single electron effect that appears in a tunnel current flowing through a very small capacitance. Further, it becomes possible to construct various logic circuits or memory circuits by using such single-electron devices.
FIGS. 1A and 1B show a basic component of a single-electron device.
Referring to FIG. 1A showing a tunnel junction, the charging energy E of the tunnel junction is given by EQU E=Q.sup.2 /2C
where C represents the capacitance of the tunnel junction and Q represents the electric charge accumulated in the tunnel junction.
When a single electron has caused a tunneling through the tunnel junction from one electrode to the other electrode, the accumulated electric charge is decreased from Q to Q-e, and the energy of the tunnel junction is changed by EQU .sub..DELTA. E=e(Q.sub.c -Q)/C
where Q.sub.c is a critical electric charge and is given by e/2(Q.sub.c =e/2).
Thus, when the electric charge Q accumulated in the tunnel junction is smaller than the foregoing critical charge Q.sub.c, such a tunneling increases the energy of the junction (.sub..DELTA. E&gt;0) and the tunneling is blocked.
When a voltage larger than e/2C is applied across the tunnel junction, on the other hand, Q becomes larger than Q.sub.c. Therefore, the tunneling of the electron is facilitated because .sub..DELTA. E&lt;0.
FIG. 1B shows the operational characteristics (I-V characteristic) of the tunnel junction. The curve of FIG. 1B clearly indicates a blocking region of current, which arises due to the foregoing single electron effect.
In order that such a single electron effect is observed in a tunnel junction, it is necessary that the energy change .sub..DELTA. E (.apprxeq.e.sup.2 /2C), which a single electron experiences when tunneling through the junction, has to have a value exceedingly larger than a thermal excitation energy k.sub.B T (e.sup.2 /2C&gt;k.sub.B T) where k.sub.B is Boltzmann's constant and T is the absolute temperature. In order to achieve the foregoing condition, it is necessary to form the tunnel junction such that the tunnel junction has a very small capacitance.
As the formation of such an extremely minute capacitor is difficult by a conventional patterning process, there have been proposals and trials to realize a minute capacitor by forming a so-called nanocrystal structure in the form of ultra-fine metal particles (metal nanocrystals) typically having a diameter of 10 nm or less. The metal particles are distributed in an insulating film such as an SiO.sub.2 film in a state that the metal particles are arranged substantially two-dimensionally with a generally identical mutual separation.
Conventionally, such a formation of the desired nanocrystal structure has been attempted by depositing ultra-fine metal particles on an insulating film by way of a sputtering or vapor phase deposition process. However, such a conventional process has been unsuccessful to form the desired metal nanocrystals, which should have a generally uniform size, isolated from each other in the insulating film, and arranged two-dimensionally in a substantially common plane in the insulating film.
On the other hand, there is a proposal to use an ion implantation process for introducing the metal elements into an insulating film for forming the metal nanocrystals in such an insulating film. According to such a process, it is possible to form the metal nanocrystals in the insulating film in the state that the metal nanocrystals are isolated from each other. See for example Hosono, et al. (Hosono, H., et al., "Cross-sectional TEM Observation of Copper-implanted SiO.sub.2 Glass," J. Non-crystalline Solids, 143, 1992, pp.157-161).
The foregoing prior art reports a successful formation of ultra-fine Cu particles in an SiO.sub.2 film by introducing Cu atoms into the SiO.sub.2 film by an ion implantation, first by setting the acceleration energy to 160 keV and conducting the ion implantation process with the dose to 6.times.10.sup.16 cm.sup.-2, followed by setting the acceleration energy to 35 keV and conducting the ion implantation process with the dose to 2.times.10.sup.16 cm.sup.-2.
However, the metal nanocrystal structure thus obtained by the foregoing prior art process shows a wide scattering of the metal nanocrystals in the depth direction of the SiO.sub.2 film, and the structure desired for a single electron device is not obtained. It should be noted that the electron has to cause a tunneling in series in such a structure when an electric field is applied across the SiO.sub.2 film. Further, the characteristic of FIG. 1B tends to change in the depth direction of the insulating film in such a scattered structure, and the observation of a clear characteristic is not possible.
In order to realize a single-electron device, it is necessary to form the metal nanocrystals in the insulating film such that the metal nanocrystals have a generally identical size and are isolated from each other with a generally uniform mutual separation. Further, the metal nanocrystals have to be arranged generally two-dimensionally in the insulating film in a more or less common plane.